bpo-29202: improve dict iteration (pythonGH-11900)

Use fewer iterations instead of iterating over the whole entry table.

Author: csabella

Objects/dictobject.c

@@ -2147,7 +2147,7 @@ dict_keys(PyDictObject *mp)
  PyObject *v;
  Py_ssize_t i, j;
  PyDictKeyEntry *ep;
- Py_ssize_t size, n, offset;
+ Py_ssize_t n, offset;
  PyObject **value_ptr;
 
  again:
@@ -2163,7 +2163,6 @@ dict_keys(PyDictObject *mp)
  goto again;
  }
  ep = DK_ENTRIES(mp->ma_keys);
- size = mp->ma_keys->dk_nentries;
  if (mp->ma_values) {
  value_ptr = mp->ma_values;
  offset = sizeof(PyObject *);
@@ -2172,7 +2171,7 @@ dict_keys(PyDictObject *mp)
  value_ptr = &ep[0].me_value;
  offset = sizeof(PyDictKeyEntry);
  }
- for (i = 0, j = 0; i < size; i++) {
+ for (i = 0, j = 0; j < n; i++) {
  if (*value_ptr != NULL) {
  PyObject *key = ep[i].me_key;
  Py_INCREF(key);
@@ -2191,7 +2190,7 @@ dict_values(PyDictObject *mp)
  PyObject *v;
  Py_ssize_t i, j;
  PyDictKeyEntry *ep;
- Py_ssize_t size, n, offset;
+ Py_ssize_t n, offset;
  PyObject **value_ptr;
 
  again:
@@ -2207,7 +2206,6 @@ dict_values(PyDictObject *mp)
  goto again;
  }
  ep = DK_ENTRIES(mp->ma_keys);
- size = mp->ma_keys->dk_nentries;
  if (mp->ma_values) {
  value_ptr = mp->ma_values;
  offset = sizeof(PyObject *);
@@ -2216,7 +2214,7 @@ dict_values(PyDictObject *mp)
  value_ptr = &ep[0].me_value;
  offset = sizeof(PyDictKeyEntry);
  }
- for (i = 0, j = 0; i < size; i++) {
+ for (i = 0, j = 0; j < n; i++) {
  PyObject *value = *value_ptr;
  value_ptr = (PyObject **)(((char *)value_ptr) + offset);
  if (value != NULL) {
@@ -2234,7 +2232,7 @@ dict_items(PyDictObject *mp)
 {
  PyObject *v;
  Py_ssize_t i, j, n;
- Py_ssize_t size, offset;
+ Py_ssize_t offset;
  PyObject *item, *key;
  PyDictKeyEntry *ep;
  PyObject **value_ptr;
@@ -2265,7 +2263,6 @@ dict_items(PyDictObject *mp)
  }
  /* Nothing we do below makes any function calls. */
  ep = DK_ENTRIES(mp->ma_keys);
- size = mp->ma_keys->dk_nentries;
  if (mp->ma_values) {
  value_ptr = mp->ma_values;
  offset = sizeof(PyObject *);
@@ -2274,7 +2271,7 @@ dict_items(PyDictObject *mp)
  value_ptr = &ep[0].me_value;
  offset = sizeof(PyDictKeyEntry);
  }
- for (i = 0, j = 0; i < size; i++) {
+ for (i = 0, j = 0; j < n; i++) {
  PyObject *value = *value_ptr;
  value_ptr = (PyObject **)(((char *)value_ptr) + offset);
  if (value != NULL) {